262 



BELL SYSTEM TECHNICAL JOURNAL 



loading coils and consideraiily longer spacing intervals than the 

 {£-28-16 system, and was consequently less expensive. The impe- 

 dance characteristics, however, were not so satisfactory at the upper 

 speech frequencies because of the greater slope of the impedance- 

 frequency characteristic, due to the lower cut-off. 



Carrier Frequency Loading. Special types of entrance cable load- 

 ing have been developed for use on incidental cables in open wire 

 lines on which carrier telephone or carrier telegraph systems are super- 

 posed. Loading system CE-4. 1-12.8 listed in Table XII exemplifies 

 this type of loading. The present standard carrier telephone systems 

 operate up to frequencies of the order of 30,000 cycles.-'* 



In order to get satisfactory impedance and attenuation character- 

 istics in the loaded incidental cables, a cut-oft' frequency of approxi- 

 mately 45,000 cycles is used. 



The highest working frequency in carrier loaded cables is approx- 

 imately 0.75 of the cut-off frequency. The ordinary mid-coil and mid- 

 section terminations do not give sufficiently close approximations to a 

 flat impedance-frequency characteristic over this wide range of fre- 

 quencies, so it has been necessary to use at the terminals of carrier 

 loaded cables, a simple impedance corrective network. 



Data regarding attenuation losses in a typical carrier loaded cable 

 are given in Table XIII. For purposes of comparison similar data 

 are given on a corresponding non-loaded cable. Effective resistance 

 values of the carrier loading coil are also included. 



The high frequency loading is used only on the side circuits, since 

 at the present time it is not customary to operate carrier telephone 

 systems over phantom circuits. The associated phantom circuit 



TABLE XIII 



Carrier Frequency Loading 



2< "Carrier Current Telephony and Telegraphy," E. H. Colpitis and O. B. Black- 

 well, Trans. A. I. E. E., Vol. 40, 1921, p. 205. 



